Blowout preventers (BOPs) are used in various petroleum operations, such as drilling or well intervention, to seal off the well during normal operations or in case of an emergency. BOPs are usually arranged in a stack of ram-type and annular preventers arranged at the wellhead, the design and operation of which is well-known to persons skilled in the art and is therefore not repeated herein.
Drilling operators are required to regularly test the BOPs in operation. This conventionally requires pulling the drill string in order to carry out the test, which is a time-consuming and thus expensive operation. Significant cost savings can be achieved by using a test valve or another ram-type BOP at the lower end of the BOP stack to seal off the wellbore around the drill string and to perform pressure tests on the BOPs higher up in the stack. This is described in Bob Judge and Gary Leach, “Subsea test valve in modified BOP cavity may help to minimize cost of required BOP testing”, Drilling Contractor magazine, November/December 2006, pp. 56-58. Additional background information is described in U.S. Pat. Nos. 6,164,619, 6,719,262 and 8,573,557.
U.S. Pat. No. 6,719,262 describes a bi-directional sealing blow out preventer. The unit described in U.S. Pat. No. 6,719,262 is capable of sealing around a drill string against fluid from below for well control as well as sealing around a drill string against fluid pressure from above for testing or pressure-activating other apparatus. The rams thereby seal against the guideway surface all around the ram bodies. No pressure equalization groove can thus exist between the front of the ram and the back as is common in conventional BOP rams. For this purpose, U.S. Pat. No. 6,719,262 describes a fluid communication system to equalize the fluid pressure between the back and the front of the ram. This is necessary in order to avoid a significant pressure differential across the ram, and thus allow the actuators to move the rams effectively.